1. Field of the Invention
This invention relates to an optical unit for use in a laser beam printer apparatus, more particularly, to an optical unit having lenses and an optical deflector which guide a light beam to an object.
2. Description of the Related Art
An optical unit for use in a laser beam printer has in general semicond laser for generating light beams, a plurality of rotatable mirrors, an optical deflector for deflecting a light beam generated from the laser, and lenses for transforming, into light beams having a desired beam spot size, the light beam directly supplied from the laser and the light beam deflected by the deflector. The lenses are sorted into a first lens group, i.e., a pre-deflection-stage optical system and a second lens group, i.e., a post-deflection-stage system.
The first lens group is interposed between the optical deflector and laser, and includes a glass or a plastic lens for transforming the light beam supplied from the laser, into a parallel beam.
The optical deflector rotates the mirrors to thereby deflect the light beam toward the object or photoconductor.
The second lens group includes focusing lens for transforming the deflected light beam into a beam having a spot size most suitable for a laser beam printer apparatus, and an f.theta. lens for which an angle .theta., at which the beam is continuously reflected by the mirrors of the deflector, is proportional to the position of the beam converged on the object in the main scanning direction.
The light beam generated from the laser is transformed into a parallel beam via the first lens group serving as a collimating lens, and then is deflected by the optical deflector in a desired direction, i.e., the light beam is reflected in accordance with rotation of the mirrors. The deflected light beam is transformed by the f.theta. lens and focusing lens, arranged in the second lens group, into a beam of a desired shape, which converges onto a desired surface portion of the photoconductor.
In the above-described optical unit, the laser is movable so that the center line of the light beam generated from the laser could be aligned with an optical axis defined between the laser and photoconductor. Further, at least one of the first and second lens groups incorporates a source-object adjusting mechanism for adjusting the length of an optical path between the laser and photoconductor.
The source-object adjusting mechanism is provided for causing the light beam from the laser to converge onto the photoconductor even when the beam spot size of the light beam is varied due to a change in the central portion of the laser, in the focal length of each lens, or in temperature or humidity. In general, a mechanism of this type includes a lens for first transforming a laser beam, which should be finally converged, into a parallel beam, and a lens for transforming the parallel beam into a converged beam. That is two or more lenses, which complimentarily act with one another, for example, they have the same power but opposite polarities, are provided between the laser and photoconductor, thereby accurately converging the laser beam onto the photoconductor.
However, if the lenses acting complementarily with one another are interposed between the laser and optical deflector, i.e., the first lens group, adjustment is necessary so as to accurately converge the laser beam onto the photoconductor, which makes it difficult to assemble the optical unit. Moreover, provision of complementary lenses inevitably makes the optical unit large.
An optical unit incorporating such complementary lenses, in which a laser beam is guided to a deflector after it is transformed into a parallel beam, is disclosed, for example, in U.S. Pat. Nos. 4,643,516 and 4,715,699.